In U.S. Pat. Nos. 5,537,041 and 5,576,624 I disclosed a metal detector apparatus which is capable of substantially reducing signals arising from ground mineralisation containing magnetic soils. Several models utilizing these principles have been commercialised and used highly successfully by gold prospectors and for land mine detection. Whilst the improvement compared to other extant metal detectors was considerable, further improvements relating to improved signal-to-noise ratio are disclosed herein which results in detection greater depth; a requirement for both prospectors and land-mine de-miners.
In U.S. Pat. No. 5,576,624 I disclosed a metal detector apparatus which transmits a pulse induction waveform and in U.S. Pat. No. 5,537,041 a rectangular voltage waveform applied to a transmit coil.
The advantages of pulse sequence containing pulses of different duration are disclosed. The historic (loss) received signal response from magnetic soils is approximately the same for all such soils (except for the overall amplitude). The instantaneous (conservative reactive) magnetic soil component is directly proportional to the transmit signal component.
In contrast the historic and instantaneous response from metal targets vary substantially.
Hence if a proportion of one part of the received signal is subtracted from a linear combination of a different part of the received signal, it is possible to select the coefficients of the said linear combination such that the response from magnetic soils are cancelled. In general, a particular said linear combination resulting in magnetic soil signal cancellation, a fist linear combination, will not result in cancellation of most metal targets, except for some particular metal targets with a particular or specific eddy current time constant decay, a first eddy current decay time constant, which happens to also result in a zero first linear combination result.
In order to detect targets with the said first eddy current decay time constant, a different linear combination of different sampled receive signals, a second linear combination, is required to cancel magnetic soil response and not cancel the said first eddy current time constant decay signal. Likewise the said second linear combination will cancel a particular metal eddy current decay signal, a second eddy current time constant decay, which will not be cancelled by the said first linear combination if chosen appropriately. Thus by simultaneously measuring and calculating at least two different linear combinations, all metal targets may be detected and magnetic soil signals approximately cancelled.
In U.S. Pat. Nos. 5,576,624 and 5,537,041 I disclosed a metal detector apparatus which is insensitive to induced e.m.f. signals from movement of a search coil relative to the earth""s magnetic field and magnetic fields from magnetic soils and rocks. This is achieved by selecting the total integrated receive signal over a complete cycle to be zero.
The problem to which this invention is directed is to improve sensitivity of a metal detector apparatus and it has for its object to provide means to improve the signal-to-noise ratio of a received signal.
In one form of this invention, this can be said to reside in a metal detector apparatus including a transmit coil adapted to transmit a magnetic field when operating with electrical current flowing through the said transmit coil, transmit timing control circuitry, power supplies, and receive signal circuitry and receive signal processing circuitry, a method:
wherein the transmit coil transmits magnetic pulses;
and the processing circuitry accumulate a receive signal which is weighted more heavily soon after the cessation of a transmit pulse and less heavily after a longer period following the cessation of a transmit pulse.
In a further form this invention could be said to reside in a metal detector apparatus of a type including a transmit coil adapted to transmit a magnetic field when operating with electrical current flowing through the said transmit coil, transmit timing control circuitry, power supply means, and receive signal circuitry and receive signal processing circuitry, wherein the transmit coil is arranged to transmit magnetic pulses, and the processing circuitry is adapted to accumulate a receive signal which is weighted more heavily soon after a cessation of a transmit pulse and less heavily after a longer period following the cessation of a transmit pulse.
All historic component signals from metal targets or magnetic soils are known to decay away following cessation of any transmit pulse or transmit signal transition. Thus the signal-to-noise implicitly becomes poorer with time after the cessation or transition of a transmit pulse. Therefore improved signal-to-noise ratio may be achieved by weighting the linear combinations greatest soon after the cessation of the transmit pulse and the weighting reduces with time.